Electrostatic Discharge (ESD) can seriously damage electronic devices and affect the operation of the systems that contain them. An article by Terry L. Welsher et al., "Design for Electrostatic Discharge (ESD) Protection In Telecommunication Products", AT&T Technical Journal, May-June 1990, pages 77-96, and an article by M-C Jon et al. "Tape and Reel Packaging-An ESD Concern", Electrical Overstress/Electrical Discharge Symposium Proceedings, EOS-10, 1988, pages 15-23, describe effects of ESD on the electronic devices. Control procedures are, therefore, required to minimize the effects of ESD. Control refers to the materials and procedures that arc employed in the manufacturing and use environments to keep static potentials and discharge currents low, below a withstand threshold of the electronic devices. Withstand threshold is the highest voltage a device can withstand without changing its operating characteristics. One such procedure is the control of the surface resistivity of the materials used for ESD related applications, such as work surfaces and tape-and-reel packages. Movement of an object, e.g., an electronic device, relative to a work surface or a pocket in a tape-and-reel package may lead to a triboelectric effect and, thus, to an ESD. The triboelectric effect may be defined as a static charge which is generated whenever two different materials come into contact and are then separated.
Surface resistivity measurements are used to classify materials into conductive, static-dissipative, and insulative categories. It is critical to correctly measure the surface resistivity of the materials used for ESD-related applications to correctly appraise the category of the material. In general, only the static-dissipative materials are recommended for the work surfaces and for the tape-and-reel packages. The need to measure the surface resistivity of a small area, such as inside of a recessed pocket of a carrier tape, is critical in ESD-safe tape-and-reel packages because: (1) each recessed pocket, by itself, is capable of discharging the triboelectric charge in the device packaged in that pocket, and (2) the pockets in a carrier tape, dependent on the process to make them conductive, could be electrically insulated from each other.
The surface resistivity (.rho.) is defined as the electrical resistance across the surface of an object measured between the opposite sides of a square on that surface, and is expressed as ohms per square (ohms/.quadrature.). The surface resistivity of a material can be obtained by measuring the surface resistance (R) between two electrodes placed on that material at the opposite sides of a square area of the material. But the measured resistance is numerically equal to the surface resistivity only if the electrodes are made to fit the sample size so that no current flows outside the electrode areas (no end-effect). To minimize the end-effect, most commercial surface resistivity meters are equipped with large rectangular electrodes, e.g., several inches long. As a result, these meters are not suitable for measuring small areas in such materials as cover tapes or inside recessed pockets of carrier tapes used to package surface mount devices in tape-and-reel packaging operation.
In a publication entitled "Standard Test Methods for D-C Resistance or Conductance of Insulating Materials", D257, pages 1-16, published by American Society for Testing and Materials, numerous test methods and apparatus are proposed. These methods are involved, requiring specific apparatus, formation of holes in the specimen, use of mercury or water as one of the electrodes, etc. Furthermore, if the measured resistance is not properly converted by the correction factor, the surface resistivity could be off by an order of magnitude or higher.
In view of the above, there is a need for a simple method and apparatus that can correctly measure the surface resistivity of areas of a material.